Standard manufacturing tolerances inherent in the threaded portions of conventionally threaded fasteners result in small air spaces between the mated male and female threads which allow the fastener combination to shift laterally or "rock" when subjected to vibration or shock. This lateral movement of the threads is sufficient to overcome starting friction between those portions of the threads which are in frictional contact with each other. When the fastener is tightened, a substantial amount of potential energy is stored in the threads (due to stresses), which is transformed in kinetic energy as the threads laterally shift with respect to each other, thereby unlocking the threads and loosening the fastener. This undesirable situation is particularly acute in assemblies which are subjected to constant vibration, such as steering assemblies, drivetrains, engines, etc.
Numerous locking thread constructions have been devised in the past in an effort to overcome the problems mentioned above. Some of these past attempts have involved the forming of the threads during the assembly process in order to increase friction between the male and female threads. For example, one typical locking thread construction is shown in my prior U.S. Pat. No. 4,150,702 which discloses a fastener combination in which conventional male threads are deformed when mated with a special female thread. Although this previous fastener combination is suitable for many applications, it is less than completely desirable in some cases since not all of the male threads mating with the female threads are deformed. Additionally, the amount of surface contact between the male and female threads is limited to that portion of the male threads which is in fact deformed, i.e., substantial flank-to-flank contact is not achieved.
Accordingly, it is a primary object of the present invention to provide a self-locking fastener which eliminates lateral or radial type movement of the threads relative to each other, in which tight frictional engagement is achieved between the male and female threads throughout the entire length of the female thread.
Another object of the invention is to provide a fastener of the type described above in which a portion of the male thread is deformed when the fastener members are assembled, but yet which also provides firm frictional engagement between one pair of opposing flanks of the male and female thread.
Another object of the invention is to provide a fastener of the type described above which eliminates the need for close manufacturing tolerances for the threads and which employs a standard male thread of constant diameter.
These, and further objects of the invention, will be made clear or will become apparent during the course of the detailed description thereof set out below.